The Coventry Diabetes Study compared the prevalence of diabetes and impaired glucose tolerance in adult Asians and Europids in relation to age, sex and body mass index. The study involved a cross-sectional house to house screening for diabetes in the electoral ward of Foleshill, Coventry, a traditional area for migration into the city. Subjects with a high blood glucose and 10 per cent of others were referred for a 75 g oral glucose tolerance test. Of the 10,304 adult residents aged 20 years or above, 3529 (64 per cent) of 5508 Europids and 3692 (84 per cent) of 4395 Asians were either screened for diabetes or already diabetic and 719 (65 per cent) of 1114 Europids and 780 (72 per cent) of 1084 Asians invited to glucose tolerance test attended. Although the prevalence of insulin-dependent diabetes was similar, the age-adjusted prevalence of non-insulin-dependent diabetes was 3.2 per cent (95 per cent confidence interval (CI): 2.6-4.0) and 4.7 per cent (CI per cent 4.0-5.5) in Europid males and females but 12.4 per cent (CI 11.0-13.8) and 11.2 CI per cent (10.0-12.5) in Asian males and females giving prevalence ratios of 3.9 per cent (3.1-5.0) in males and 2.4 per cent (2.0-2.9) in females. These differences were not due to differences in body mass index. The prevalence of impaired glucose tolerance was also higher in Asians aged below 60 years, and in 65 per cent of Europids and 40 per cent of Asians non-insulin-dependent diabetes was previously undiagnosed. The non-insulin-dependent diabetes/impaired glucose tolerance ratio was significantly higher in Asians than Europids. Non-insulin-dependent diabetes in Asians differs from that in Europids. Besides the higher overall prevalence, there is a greater proportion of males, a lower proportion undiagnosed disease, a younger age at diagnosis and a greater proportion of abnormal glucose tolerance that is due to non-insulin-dependent diabetes.